54 ; ’ INVERTEBRATE ANIMALS. 
SUB-KINGDOM II. CGQELENTERATA. 
CHAPTER IV. 
I, CHARACTERS OF THE SuB-KINGDOM. 2. DIVISIONS. 
3. GENERAL CHARACTERS OF THE HyYDROZOA. 
4. EXPLANATION OF TECHNICAL TERMS. 
In the sub-kingdom Celenterata are included the sea-anem- 
ones, corals, sea-jellies, sea-firs, and other allied animals, and 
the whole division may be looked upon as forming the most 
typical section of the animals formerly called by Cuvier Radiata. 
In addition, however, to the above-mentioned animals, Cuvier 
included in his Radiata all the members of the modern sub- 
kingdom Protozoa, together with the sea-mats or lowest class 
of the JZod/usca, and the sea-urchins, star-fishes, wheel-animal- 
cules, internal parasites, and others which are now placed in 
a separate sub-kingdom by themselves (Aznulotda). The old 
fadiata, therefore, was an extremely heterogeneous assemblage, 
and there is no advantage to be derived from its employment 
even in works such as this present. The division Celenterata, 
or ‘“ hollow-entrailed ” animals (Gr. 4ozZos, hollow ; and enteron, 
intestine), includes all those radiate animals which are more or 
less closely allied to the sea-anemones on the one hand, and 
to the sea-firs on the other. Most of the Celenterata come 
under the conveniently loose term of “ zoophytes,” or plant- 
animals, from the external resemblance which many of them 
show to plants. | 
The Celenterata may be defined as animals whose alimentary 
canal communicates freely with the general cavity of the body 
(“somatic cavity”). The body is essentially composed of two 
layers or membranes, an outer layer or “ ectoderm,’ and an inner 
layer or “endoderm.” No circulatory organs exist, and in most 
there are no traces of a nervous system. Peculiar stinging organs, 
